1. Field of the Invention
The invention relates to a rolling bearing arrangement for an electromotor having a rotor rotatably arranged inside a stator, including a bearing seat which is fixed with respect to the stator, and a bearing with an outer race, an inner race, and a plurality of rolling bodies between the races, the outer race being received in the bearing seat, the inner race being arranged on the rotor shaft.
2. Description of the Related Art
Particularly in the case of asynchronous motors, in conjunction with applications where a constant speed prevails, a flow of current occurs between the outer stator and the rotor borne rotatably therein, the circuit being closed by the rolling bearings for bearing the rotor. The flow of current causes damage to the rolling bearings. Current-damaged bearings have typical transverse corrugations on the raceway that lead to vibrations and noise before the bearing finally fails. The flow of current through the bearing takes place from the inner ring thereof to the outer ring. In this case, the current must flow through the rolling body arrangement—mostly lifetime lubricated steel balls—and must overcome on this path two lubricant films, specifically on the part of the inner ring and on the part of the outer ring. The electric strength of the lubricant films, connected in series to this extent, of the rolling bearing is stationary at about 0.5 volts measured from the inner ring to the outer ring. Consequently, given a voltage of 20 volts across the rolling bearing it is possible to hold for only approximately 100 microseconds before a breakdown occurs. For the rest, the breakdown voltage is also a function of the speed, the temperature and other parameters. However, what is decisive is that a bearing current is always associated with an electric breakdown of the lubricant film. The damage resulting therefrom to the raceway happens according to a principle that can be compared with spark erosion.
A bearing current is produced in the case of a three-phase AC motor by the asymmetric field distribution. The result of this is that the rotating field induces a small voltage of supply frequency in the shaft of the rotor. The insulating lubricant film of the rolling bearings is broken down starting from a critical value, and said circuit, which flows through the rotor, via rolling bearings on both sides and the stator, is formed. Furthermore, capacitive discharge currents also occur between the rotor and the stator, or pulse-type circulating currents occur that are associated with a converter operation.
It has already been attempted to avoid damaging bearing currents by using a conductive grease in the region of the rolling bearing. Although conductive constituents—such as graphite—is added to this grease, the experiment showed no important change in the insulating properties of the lubricant film, and so an electric breakdown of the lubricant film with damage to the raceway cannot be prevented thereby.
Solutions that effect suppression means filter arrangements or completely interrupt the circuit in the electrometer that damages the rolling bearings have gained acceptance in practice for the purpose of avoiding a bearing current. This latter is achieved in a simple way by using an insulated bearing. Available therefor in the prior art are either bearings with an insulating layer on the outer ring, or bearings with ceramic rolling bodies.
In the case of a thin insulating layer arranged on the outer ring, the bearing current can, however, not be completely suppressed, because the insulating layer—consisting mostly of aluminum oxide—constitutes an electric capacitor that remains permeable to high-frequency components of a bearing current. Furthermore, it is decidedly expensive to apply the insulating layer.
In the case of bearings with ceramic rolling bodies, so-called hybrid bearings, it is possible, by contrast, to achieve a reliable insulation effect such that the flow of current through the rolling bearings can be effectively interrupted. Such hybrid bearings are, however, expensive to produce and result to this extent in comparatively high material costs. Moreover, it is disadvantageous that hybrid bearings are not available in all standardized sizes.